It is a compile-time option so currently no. As xyz321 says, you might experience problems if you run it on a non-standard port unless you exclusively use passive mode. Can your router not do a port rewrite as part of its address translation? (most can)
Ok, if it cannot be done without a compile then I understand. I ran a server for many years on a high ftp port number and of course forwarded the port. I forward port 21 now to access the humax. I am not sure if my router can translate the port address but will investigate. I doubt it as I have seen no option other than a direct forward. I have experience of other routers and I agree, they could.
I feel I cannot ask you to do that. Most people will use the server from the LAN so it would be unfair for me to ask you to do this for the minority. What I would ask is that if you have occasion to recompile then you consider it at that time. Your work here is much appreciated and I don't want to increase the burden.
My router has a 'port triggering' option but I don't think that is suitable for ftp although I stand to be corrected.